1. Field of the Invention
The invention concerns a method to operate a magnetic resonance tomography system (MT system) to generate magnetic resonance image data of an examination subject, of the type wherein a number of slices in the examination subject are excited within a sequence module by respective RF slice excitation pulses of a series of a spatially selective RF slice excitation pulses, and then multiple RF refocusing pulses are emitted, and wherein the width of the RF refocusing pulses for generation of a number of time-separated echo signals is selected per RF refocusing pulse such that it encompasses at least a portion of an excitation volume of all excited slices for simultaneous refocusing of all excited slices. Furthermore, the invention concerns a method to generate magnetic resonance image data of an examination subject, wherein raw data are used that were acquired by a magnetic resonance tomography system using such a method. Moreover, the invention concerns a control device for a magnetic resonance tomography system to implement such a method, as well as a magnetic resonance tomography system with such a control device.
2. Description of the Prior Art
In a magnetic resonance system, the body to be examined is typically exposed to a relatively strong basic magnetic field (for example 1.5 Tesla, 3 Tesla or 7 Tesla) produced by a basic magnetic field system. After application of the basic field, nuclei in the examination subject with a non-vanishing nuclear magnetic dipole moment (frequently also called spin) align along the field. This collective response of the spin system is described with the macroscopic “magnetization”. The macroscopic magnetization is the vector sum of all microscopic magnetic moments in the subject at a specific location. In addition to the basic field, a magnetic field gradient with which the magnetic resonance frequency (Larmor frequency) is determined at the respective location is additionally applied by a gradient system. From a radio-frequency transmission system, radio-frequency excitation signals (RF pulses) are then emitted by suitable antenna devices, which lead to the nuclear spins of specific nuclei being excited to resonance (i.e. at the Larmor frequency present at the respective location) by this radio-frequency field by being flipped by a defined flip angle relative to the magnetic field lines of the basic magnetic field. If such an RF pulse acts on spins that are already excited, these can be flipped into another angle position or even be flipped back into an initial state parallel to the basic magnetic field. During the relaxation time of the excited nuclear spins, radio-frequency signals (known as magnetic resonance signals) are radiated upon resonance, and these magnetic resonance signals are received by suitable reception antennas, and then are processed further. The acquisition of the magnetic resonance signals takes place in the spatial frequency domain (known as “k-space”). K-space is traversed (i.e., the acquired data are entered therein at points) over time along a “gradient trajectory” (also called a “k-space trajectory”) defined by the switching of the gradient pulses during a measurement of a slice, for example. Moreover, the RF pulses must be emitted in coordination, matching in time. Finally, the desired image data can be reconstructed by means of a two-dimensional Fourier transformation from the “raw data” acquired in such a manner.
Defined, predetermined pulse sequences (i.e. sequences of defined RF pulses and gradient pulses in different directions and of readout windows during which the reception antennas are switched to receive and the magnetic resonance signals are received and processed) are typically used to control a magnetic resonance tomography system in the measurement. In a measurement protocol, these sequences are parameterized in advance for a desired examination, for example a defined contrast of the calculated images. The measurement protocol can also include additional control data for the measurement. There are a number of magnetic resonance sequence techniques according to which pulse sequences can be designed. One of the major challenges in the future development in magnetic resonance imaging is an acceleration of magnetic resonance sequence techniques without wide-ranging compromises with regard to resolution, contrast and tendency towards artifacts. An increase of the examination speed leads to a smaller exposure time of the patient, who must remain at rest within the (most often quite narrow) magnetic resonance tomography scanner over a longer period of time. Because the applications and possibilities of magnetic resonance imaging are diversifying and thus the number of measurement protocols available to be executed per examination grows, a reduction of the measurement time for individual measurement protocols becomes all the more important. In addition, the examination duration of an MR examination is directly linked with the patient throughput, and therefore the examination costs. In order to increase the number of patients who can be helped with an MR examination, and given the background of increasing costs in health care systems, an aging population in the highly industrialized nations and the desire to also make magnetic resonance imaging accessible to people in less highly developed countries, this is also an important aspect for the acceleration of the individual measurements.
The measurement time per measurement protocol could already be drastically reduced in part with the integration of fast sequence techniques—such as “Turbo Spin Echo” (TSE) sequences or, respectively, “Fast Spin Echo” (FSE) or “Echoplanar Imaging” (EPI)—and what are known as parallel acquisition techniques into the clinical routine. An example of an FSE sequence is described in U.S. Pat. No. 6,771,069 B2.
TSE sequences use an RF excitation pulse, followed a series of RF refocusing pulses. The spin echo arising after each refocusing pulse is normally individually phase-coded, such that multiple k-space lines can be acquired per excitation, and thus the acquisition time is reduced relative to classical spin echo sequences. The TSE technique and the FSE technique are especially important in clinical diagnostics—in particular for the T2 contrast—due to its relative insensitivity with regard to off-resonance (i.e. a deviation from the Larmor frequency), which can occur, for example, as a result of system imperfections, magnetic susceptibility variations of the tissue, metallic implants, etc. For special variants of these sequences, separate acronyms are used, such as “Rapid Acquisition with Relaxation Enhancement” (RARE), “Half-Fourier Acquired Single-shot Turbo Spin Echo” (HASTE) and “Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction” (PROPELLER), which is explained below. A TSE sequence technique, however, is a relatively slow sequence technique in comparison to EPI techniques and is characterized by high radio-frequency radiation into the patients due to the large number of refocusing pulses. The specific absorption rate (SAR)—thus the radio-frequency energy that is absorbed in a defined time interval per kg of body weight—is regulated. This has the effect that the acquisition time of a TSE sequence—in particular at field strengths of 3 Tesla or more—is normally not limited by the capacity of the MR system (for example the gradient system) but rather by the specific absorption rate. In systems known as ultra-high field systems with field strengths of 7T and beyond, an examination with a TSE sequence—with a slice count sufficient for the coverage of the anatomy to be examined and in a clinically acceptable measurement time—has previously not been possible due to the associated SAR exposure.
In order to achieve an additional acceleration, in a relatively new group of acceleration techniques (SMA—“Simultaneous Multi-Slice Acquisition”) that have not yet become established in clinical practice. In such techniques, it is sought to excite multiple slices of a slice stack either simultaneously (by means of “wideband MRI”) or in a short time series (designated as “Simultaneous Echo Refocusing”), and then either to separate the signal emitted from the different slices as a result of this excitation into readout windows in (close) chronological succession or to simultaneously receive said signal and subsequently separate it via suitable post-processing methods (in “post-processing”).
In principle, it would be desirable to also excite multiple slices of a slice stack simultaneously or in a short chronological series, and to simultaneously refocus them repeatedly within the scope of a TSE sequence technique, as mentioned above. Due to the cited SAR problem, however, such a new TSE sequence technique with a simultaneous acquisition of multiple slices will then only be able to shorten the actual examination duration when the radio-frequency radiation at least does not increase per time period. This fact makes a series of new SMA techniques for TSE sequences practically irrelevant. An additional difficulty in the design of such novel TSE sequences is the fact that the refocusing pulses are normally not perfect 180° pulses. An inherent, unavoidable reason for this is that the slice profile is not exactly rectangular (due to the finite duration of the RF pulses), and thus deviates from the ideal 180° at least at the slice edges.
As a result, the “refocusing pulse” only partially refocuses the existing transversal magnetization, flips a portion of the remaining, unfocused magnetization back in the longitudinal axis, and leaves the rest unaffected. The longitudinal magnetization (i.e. the magnetization proceeding in the direction of the basic magnetic field) that is present before the “refocusing pulse” is accordingly partially “excited”, partially inverted and partially left unaffected in the transverse plane. The transverse magnetization that is present after the “refocusing pulse” (i.e. the spins that are currently excited) then accumulates as a result of the switched gradient fields and/or a phase portion accumulates as a result of unwanted, off-resonances that may be present, while the longitudinal magnetization is unaffected by the switched gradient fields and is subject only to the relatively slow T1 decay until it is flipped back by one of the following “refocusing pulses” in the transversal plane.
Each “refocusing pulse” thus acts as an inversion pulse for one portion of the spins; as an excitation pulse for a different portion; as a “restore pulse” (which flips the spins back in the longitudinal direction, wherein the current phase positions of the spins is maintained) for an additional portion; and is transparent for the remainder. Spins on which each “refocusing pulse” acts similarly follow what is known as a coherent echo path. The number of different coherent echo paths increases exponentially with the number of refocusing pulses. Spins that result from different coherent echo paths normally contribute to a signal that is acquired as of the second “refocusing pulse” in a readout window. If these spins accumulate along the different coherent echo paths of different phase portions, this leads to destructive interference. The signal collapses; the images calculated from the raw data show shadows and a poor signal-to-noise ratio (SNR); and the pulse sequence is not able to maintain a long echo train. The latter is a requirement for the T2 contrast (which is particularly important in connection with the TSE imaging) and the efficiency increase, which can be achieved relative to a spin echo sequence.
In order to ensure that only those coherent echo paths along which the spins accumulate the same phase portions contribute to the signal in each readout window, in the article “Simultaneous Spin-Echo Refocusing” in Magnetic Resonance in Medicine, 54, 2005, P. 513-523 by M. Günther and D. A. Feinberg, and in U.S. Pat. No. 6,853,188 B2, a TSE sequence is described in which m adjacent slices are excited in short time intervals and respective echoes of the m slices are refocused with a series of refocusing pulses. With a specific scheme with spoiler gradient pulses, echoes whose signal is emitted by spins of different slices are prevented from undesirably coinciding in a readout window. This spoiler scheme dephases signals of those spins that follow specific, coherent echo paths. The simultaneous refocusing of the m slices causes the radiated radio-frequency energy to be reduced by approximately a factor of m. However, due to the spoiling of specific, coherent echo paths, the signal linked with these echo paths cannot be used for imaging, which leads to an SNR loss relative to the separate acquisition of the slices. In addition, in this pulse sequence it is unfortunately not possible to maintain a long echo time (for example with more than 20 echoes). Therefore, the pulse sequence cannot be used for T2-weighted imaging.
Therefore, in EP 2 239 592 A1 a RARE sequence is proposed in which the refocusing of multiple excited slices is achieved separately by slice-selective refocusing pulses in short succession. Particularly when a larger number of slices should be excited simultaneously, this separate refocusing leads to a not-insignificant lengthening of the sequence.